The goal of the proposed research is to determine how higher plants signal the arrest of a critical developmental pathway in the reproductive cycle. The experimental system is that of receptor- mediated signalling in the self-incompatibility response of Brassica, a genetically controlled self recognition system that prevents self- fertilization by arresting the development of genetically related pollen grains. Our cloning and characterization in 1991 of the receptor serine/threonine kinase that mediates this self-recognition, soon after the description in 1990 of the first member of the TGF-beta class of receptors with serine/threonine specificity, represented an important advance in our understanding of self-incompatibility and of cell-cell communication in plants. Our recent identification of a gene with sequence similarity to aquaporins (water channels) as an effector of receptor-mediated signalling in this system provides the first molecular evidence relating to the mechanism by which the activation and development of pollen is arrested. We propose to characterize this water channel, its cellular and subcellular distribution in plant cells, define its biophysical and biochemical properties, and determine its role in the arrest of pollen. In addition, we plan to identify other components of signal transduction in this system and construct a biochemical pathway that links the signalling receptor to activation of its putative downstream target, the water channel. When considered in light of work on receptors and water channels in hormone-induced activation of osmotic water permeability in mammalian cells, this study should provide for an informative comparison of the mechanisms by which the regulation of transcellular water transfer is achieved in organisms as divergent as plants and mammals.